Transcript No Slide Title

2007 Michell Medal Oration
F-111 Structural Integrity Support
Francis Rose
Chief Scientist, Platforms Sciences Lab, DSTO
OUTLINE
• Michell Brothers
• F-111 Sole Operator Program
• Hole Shape Optimisation
• Bonded Repair Substantiation
• Loose Ends & Acknowledgements
Anthony George Maldon Michell 1870-1959
Michell Structures
Tilting-pad Thrust-bearing
John Henry Michell 1863-1940
The Wave Resistance of a
Ship
Stress Compatibility
Equations
Michell Brothers Legacy
• Contributions to both Fluid & Solid Mechanics
• Application Driven
• Uncompromising Intellectual Integrity
& Quality of Engineering Science
“Theory is the captain; practice the soldiers”
F-111 SOLE OPERATOR PROGRAM
• Background
• Hole Shape Optimisation
• Bonded Repair Substantiation
F-111 SOP BACKGROUND
• USAF (1967-1996) & RAAF (1973-2010)
• USAF Early Retirement Announced Dec 1994
• RAAF Supportability Study 1995 – 96
• DSTO to address
• Engineering Risk
• Ageing Aircraft Risk
INNOVATIONS, ACCIDENTS & WATERSHEDS
• de Havilland Comet (1953-54)
• General Dynamics F-111 (1969)
• Aloha Airlines Boeing 737 (1988)
F-111 SWING WING MECHANISM
CRACKING IN THE WING PIVOT FITTING
Fuel Flow Vent Holes (FFVHs)
Stiffener Runouts (SROs)
Typical crack
Typical crack
Inside WPF upper plate
FUEL VENT HOLES: WEIGHT REDUCTION PROGRAM
in-service fatigue cracking
FFVH 14
FFVH 13
FFVH 11
FFVH 12
HOLE SHAPE OPTIMISATION
Initial hole
4
Optimal hole
s1
3
s1
s3
Stress
2
1
0
-1
s4
s2
-2
0
20
40
60
80
100
% arc length around boundary
•
Optimal hole characterised by (piecewise) constant hoop stress
•
Iterative boundary deformation to achieve constant hoop stress
j
j


σ

σ
i
th 
j

di 
C
 σ thj



σ thj  max σ ij
ITERATIVE BOUNDARY DEFORMATION
(constraint: only material removal allowed, multi-peak stress minimisation)
S2
e
Vertical
constraint
line
y
H/2
h
x
w
H/2
W
S2
•
•
•
Initial 2:1 elliptical hole
2:1 Optimal hole
21% reduction in peak stress compared to an initial elliptical hole
43% reduction in peak stress compared to a circular hole
Greater stress reduction with increasing hole aspect ratio
INITIAL AND FINAL STRESS
(constraint: only material removal allowed, multi-peak stress minimisation)
S2
Initial 2:1 ellipse
4
e
s1
3
y
H/2
h
x
w
1
0
H/2
W
-1
s2
-2
0
S2
s1
s3
2
Stress
Vertical
constraint
line
Optimal hole
20
40
s4
60
80
100
% arc length around boundary
Uniform stress regions are very flat, indicating true optimality.
(20% & 6% reduction in maximum +ve peaks, 22% reduction in –ve peaks)
FE Implementation
 Only move nodes on one edge of a mesh generation block (B1, B2)
 New element mesh created for each iteration (avoids mesh distortion)
 It is also useful to maintain relative spacing of boundary nodes.
BENEFIT FOR INSPECTION INTERVAL
Inspection interval (hours)
8000
6000
4000
estimated inspection
interval trend
new
position
2000
current
position
0
1000
2000
3000
4000
Peak von Mises stress (MPa)
5000
TOOLING FOR RE-WORK
Electrode plate
Finishing
electrode
Roughing
electrode
Locating probe
– optimal reworks
manufactured into a
test wing by electro
discharge machining
NEXT MOST CRITICAL LOCATIONS
FFVH 14
FFVH 13
FFVH 11
FFVH 12
WING DAMAGE ENHANCEMENT
– Static tests are used to validate FE model
– Cyclic test results are interpreted for Durability and Damage Tolerance
BUCKLING ANALYSIS OF WING PIVOT FITTING
CPLT Load:
Blueprint configuration
REPAIR SUBSTANTIATION
REPAIR SUBSTANTIATION REQUIREMENTS
• Validation of design analysis by an independent
method
• Validation testing of a representative test article for
Static strength
Durability and Damage Tolerance
Proper accounting for environmental effects
LOAD FLOW & LOAD TRANSFER
CRACK LOCATION
LOCAL GEOMETRY
FRACTOGRAPHY OF CRACKING
PANEL SPECIMEN
BOX SPECIMEN
BOX SPECIMEN TESTING
FATIGUE CRACK GROWTH COMPARISON
Panel specimen crack growth
under cycle-by-cycle spectrum loads
80
Crack length
tip-to-tip (mm)
75
70
UNPATCHED
PANELS
65
60
PATCHED
PANELS
55
50
45
40
0
10,000
20,000
30,000
40,000
50,000
Equivalent flight hours
RESIDUAL STRENGTH RESULTS
D.U.L
D.L.L
Unpatched, Uncracked
Unpatched, Cracked, RT
2a = 40 mm
Patched, Cracked, RT
2a = 40 mm
Patched, Cracked, +110C
2a = 40 mm
Patched, Cracked, -40C
2a = 40 mm
Patched, Cracked After
30,000 Flight Hours
2a = 63 mm
0
50
100
150
200
250
300
FAILURE STRESS (MPa)
350
400
450
FEATURES OF MECHANICAL REPAIRS
Repair

Filler
Stringer



Skin
New Crack
Doubler


New stress concentrations at
fastener holes
Difficult to detect cracks under
patch
Low patching efficiency,
cannot patch cracks
May damage hidden
components
May cause corrosion problems
Simple to apply - no new
technology
FEATURES OF BONDED REPAIRS
Repair


Stringer


Original Crack
Skin
Doubler


No damage to structure or
hidden components
High patching efficiency, can
repair cracks
Can detect cracking under
boron/epoxy patch
Minimises stress
concentrations
No corrosion problems
Simple/effective surface
treatment essential